Snowmobile track suspensions employ linkage arms and springs between the chassis tunnel and the skid-frame rails, which provide the path for track rotation and energy transfer from the ground to the chassis. In one typical arrangement trailing front and rear arm linkages are pivotally secured at their front (upper) ends to the chassis tunnel. The rear (lower) ends of the linkage arms are pivotally secured to the skid-frame rails. At the rear, lower end the attachment is also through a loss-motion linkage connection. A spring is typically coupled between the tunnel and the skid frame adjacent to the rear link. A spring is also secured between the upper end of the front arm and the skid frame. Essentially, this arrangement allows for no pitch control. With the addition of the coupling system, a system that couples the rear and front suspension arms together, a sprung four-bar-linkage arrangement is created. Thus, when either of the arms is pivoted due to a bump (for example) the chassis tunnel is shifted forwardly relative to the skid-frame rails and the other arm is thereby pivoted, compressing the entire suspension. Such compression creates loss of traction and can create a rougher ride as the suspension does not conform well to terrain transitions. For example, if the rear end of the track is on a bump, the track front is also raised, releasing it from positive traction on the snow surface.
Some attenuation of this four-bar-linkage effect is achieved as the rear arm actually comprises two interconnected linkages: a rear arm and an idler arm. This rear/idler arm arrangement allows some suspension compression at the rear end of the track without forward shifting of the tunnel relative to the rails. However, to prevent the front end of the snowmobile from excessive lift under strong acceleration, rear arm coupler blocks limit movement of the rear arm. Once the blocks stop the rear arm movement, further compression of the suspension causes the idler arm to move the tunnel forward relative to the rails. Such forward tunnel movement also pushes the front arm into a more laid down position, compressing the front track suspension. Since both springs are compressed during such suspension action, attempts to solve the problem have focused on the provision of a softer front spring to keep the overall suspension stiffness within an acceptable range.
The problem also limits coupler block settings to achieve desirable suspension action. For quick, stable acceleration, a close coupler block setting is needed. However, if the blocks are set too close, the rear arm range of motion is excessively limited and the idler arm compresses the whole suspension by pushing the chassis forward relative to the skid frame as discussed above.
A further related problem is encountered once the idler arm is compressed to the point that it goes over-center. The arm may be temporarily locked into the compressed state by the upward force on the front arm. The tunnel connection to the front end of the idler arm pushes the arm downward, as the front spring resists the compression that would have to occur to allow the idler arm to lift. Since the idler arm is over-center, initial lifting of the arm would cause the chassis to move forward relative to the skid-frame—compressing the track front suspension—until the idler arm crosses back from the over-center position.
In certain suspension configurations, full rear track suspension action is limited by the front arm arrangement. Once the front shock is compressed to the point that the front arm is parallel to the rails further rear suspension travel is stopped. In this parallel position, the tunnel is not able to shift relative to the rails. Therefore, the rear idler arm cannot further move, as such movement would require the tunnel to shift forward.
Due to the drawbacks inherent in the current suspension arrangements, a system for improved traction, steering, and bump absorption is needed.